Modern telecommunications networks often employ IP networks to route data and voice traffic. In many situations, an IP network for serving a number of users may be distributed over a wide geographic area, such that users are not necessarily in the same city, state, or even country. All of the voice call traffic may be routed and managed by a central call server in the IP network. The central call server may be in a different physical location from many of the terminals connecting to the IP network. Edge gateways connect the IP network to the PSTN, allowing calls originating in the IP network to reach termination points in the PSTN, and vice versa.
Traditional TDM sets, i.e. digital and analog sets that are not IP-enabled, are still used even in the context of IP networks for telephony. The IP network includes IP media gateways (IPMG) for connecting TDM sets to the network. The central call server manages the routing of calls from the IPMG to termination points.
In some circumstances, it would be desirable to base the call routing on the location of the TDM set.
For example, in the traditional public-switched telephone network (PSTN), an ESA call is placed over a physical circuit from a TDM phone to a private or public branch exchange or switch. The switch or exchange recognizes the nature of the ESA call, e.g. an E911 call in North America, and routes the call to the local public service answering point (PSAP). The TDM phone, the exchange or switch, and the PSAP are all co-located in the same general geographic area. Accordingly, a user calling 911 is assured of reaching a PSAP in his or her general area.
A difficulty that arises with ESA calls in an IP network configuration is that the central call server and the edge gateways may be located in different geographic regions from the user of the TDM set that places an ESA call. Accordingly, if the central call server simply routes an ESA call to an edge gateway, the call will enter the PSTN at a geographic location that may bear no relation to the geographic location of the caller. As a result the caller may be connected with a PSAP in a different city, state, or even country.
In another example, a TDM set may place a call to a PSTN location within the local area code of the TDM set, i.e. a local call. In some circumstances, the central call server may be located in another area code, i.e. in another city or state or country. As a result, the call from the TDM set may enter the PSTN at a location that would render the call a long-distance call through PSTN circuits.